Monolayer plastic films based on a vinyliden chloride film have been used as a casing for processed meat and fish foods, dairy products such as cheese and butter as well as for confectionaries such as jelly. This casing is produced by forming a plastic film into a tubular form so that top and rear surface of the film will be overlaid on each other along the length direction. The overlaid portion is sealed in a manner similar to an envelope being sealed. A heat seal section (a lug section) will be left, with the two edges thereof bound. A metal wire may be used for binding ends or the sections may be sealed by means of an ordinary sealing method.
When opening this casing, there are some difficulties such as cutting a portion of the film with such a tool as a knife and peeling the film off from the section. To overcome this difficulty in opening the casing, there have been proposed various methods to improve the ease of opening the casing. The proposed methods include, for instance, the method in which a strip film stronger than the base film is fusion-bonded to a portion of the base film or the sealed section and the base film is broken by pulling the strip film. Another method involves a V-shaped notch or an I-shaped notch provided as an unsealing means in the portion of the base film not heat-sealed yet and the base film is torn from the notch. Another method involves a number of flaws intentionally provided in a section not heat-sealed yet and the base film is torn from the section.
A trend, in the field of casing film for particular applications, is the consideration given as to a customer's demands for non-chloride-based plastic films. This demand for non-chloride-based plastic films will increase in the future. The casing based on this non-chloride-based plastic film is used in the same way as the vinyliden chloride film described above. However, the capabilities as to function, as a gas barrier and a heat seal are not as required using a non-chloride-based film only. It is therefore generally considered that the mono layer plastic film can not satisfy most needs. The possibility of developing a composite film using the non-chloride film as a component thereof has been investigated. For the reasons as described above, the opening means available for the vinyliden chloride monolayer film casing as described above is insufficient for giving the adaptability to be easily opened to this casing based on the non-chloride-based film.
The above-described technology for easily opening a casing can be employed for the vinyliden chloride film itself because the film is adapted to be easily torn. However, when a composite plastic film is used in place of the monolayer vinyliden chloride film, resistance of the composite plastic film to tearing is high, and it is difficult to open a casing by tearing the film itself.
As described above, the technology for easily opening a casing based on the composite plastic film has not been established.